1. Filed of the Invention
The present invention relates to a recording medium having air-sandwich structure, and more particularly to a recording medium of air-sandwich structure for writing and/or reading information by means of laser beam of high energy density.
2. Description of Prior Arts
Information recording media utilizing a beam of high energy density such as laser beam have been developed in recent years and are put to practical use. Such recording medium is called an optical disc and the practical applications thereof have been found, for example, as video disc and audio disc as well as disc memory for large-capacity computer and large-capacity static image file.
The optical disc basically comprises a disc-shaped transparent substrate of a plastic or glass material and a recording layer composed of a metal such as Bi, Sn, In or Te or a semi-metal provided on said substrate. Writing of information on the optical disc can be conducted, for example, by irradiating the optical disc with laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the optical disc absorbs energy of the beam and a rise in temperature locally occurs and as a result, a chemical or physical change is caused to alter optical characteristics of the recording layer in the irradiated area, whereby the recording of information can be made. Reading of information from the optical disc is also conducted by irradiating the optical disc with laser beam. The information can be reproduced by detecting reflected light or transmitted light corresponding to the alteration in the optical characteristics of the recording layer.
As a disc structure for protecting the recording layer, there has been recently proposed an air-sandwich structure in which a recording layer is provided on at least one of two disc-shaped substrates and positioned inside the two substrates opposed to each other at a distance by interposing a ring-shaped inner spacer and a ring-shaped outer spacer between the substrates so as to form a closed space surrounded by the two substrates and two spacers. The spacer is made of a rigid material such as aluminum or iron. In the optical disc having such structure, the recording layer is not directly exposed to outer air and the recording and reproduction of information is conducted with laser beam passing through the transparent substrate. Accordingly, there is little possibility that the recording layer is physically or chemically damaged or the surface is covered with dust which is to interfere with the recording and reproduction of information of the recording medium.
A typical structure of the conventional recording medium of air-sandwich structure is illustrated in FIGS. 1 and 2, wherein FIG. 1 schematically shows an appearance of the recording medium and FIG. 2 shows a constitution of the medium by way of a sectional view.
In FIG. 2, on each of disc-shaped substrates 21a, 21b was provided each of recording layers 22a, 22b. These substrates are bonded to each other by ring-shaped inner spacer 24 via adhesive layers 23a, 23b and ring-shaped outer spacer 26 via adhesive layers 25a, 25b, whereby a closed inner space 27 is formed by the two substates 21a, 21b, the inner spacer 24 and the outer spacer 26.
As is described above, the substrates and the spacers are bonded to each other by an adhesive, which is apt to overflow towards both sides of the spacers. In more detail, an excessive amount of an adhesive is generally supplied between the substrate and the spacer to securely fix the spacer to the substrate, whereby increasing a mechanical strength of the air-sandwich structure. The excessively supplied adhesive is forced to flow out on the both sides of the spacer in the course of a procedure for bonding the spacer and the substrate proceeds under pressure. The adhesive having flowed out on both sides of the outer spacer as well as the adhesive having flowed out on the outer side (i.e., side of closed inner space 27) of the inner spacer both cause substantially no problems in practice. However, the adhesive having flowed out on the inner side (where the recording medium is to contact a spindle for rotating the medium in the procedure for writing or reading the information) causes troubles because the adhesive having flowed out and thus protruding towards the inner side causes variation of the size of the inside diameter of the recording medium. The varied size of the inside diameter of the recording medium brings about troubles in chucking with a spindle. Othewise, the varied size of the inside diameter of the medium causes eccentric rotation resulting in troubles in tracking.
In more detail, the inner wall of the inner spacer of the conventional optical disc is kept in contact with the spindle during the rotation of the disc. The writing and reading of information are carried out when the disc rotates around the spindle with high precision. Accordingly, the inside diameter of the inner spacer is very severely defined. Thus, the adhesive which protrudes from the inner wall of the inner spacer reduces the precision of the inside diameter, whereby reducing the precision in the writing and reading characteristics. Therefore, the protruded adhesive is necessarily cut off. Such cutting procedure requires additional works and sometimes causes other eccentricity.
For obviating the above-mentioned problems in the conventional recording medium of air-sandwich type, Japanese Patent Provisional Publication No. 59(1984)-14153 proposes that the the inner spacer be made of a material having increased mechanical strength and abrasion resistance as compared with the substrates and that the inside diameter of the inner spacer be made smaller than the inside diameters of the substrates. From one aspect, the proposed recording medium is advantageous in assembling, because the inside diameter of the recording medium is be defined only by the inner spacer. From other aspect, however, the proposed recording medium is disadvantageous because it necessarily uses a material of high strength such as iron or aluminum, because stable rotation required for the recording medium is needed to be assured only by the inner spacer. Such metal spacer is not economical. Further, the metal spacer is sometimes not appropriately employed in the case that plastic or glass substrates are employed. In more detail, the metal spacer and the plastic or glass substrates are substantially different in the coefficient of thermal expansion and the coefficient of expansion upon absorption of vaporized water, and such difference brings about peeling off and cracking of the adhesive layer through repeated application of shearing force when the recording medium is stored or employed at a high temperature and high relative humidity.
Japanese Patent Provisional Publication No. 60(1985)-103535 proposes that the troubles due to the overflow of the adhesive be removed by chamferring the substrates at the area for receiving the adhesive. However, a recording medium of this type easily receives on the surface of the substrates dust produced in the chamferring processing. The dust deposited on the surface of the substrate ought to be washed off after the chamferring processing. The requirement of such additional stage is naturally not advanageous.